The invention relates to a process for controlling the deposition of, in particular, calcium salts during the evaporation of sugar streams.
In sugar production deposition of calcium salts (xe2x80x9cscalexe2x80x9d) takes place during evaporation, Said scale consists of fluctuating mixtures of calcium carbonate, calcium oxalate, calcium phosphate, silicates and organic substances and occurs both in the sugar beet and the sugar cane industry. The consequence of this scale is that heat exchange proceeds less well, with a reduction in the efficiency of the evaporation as a result. In the past, the calcium scale therefore had to be removed by mechanical cleaning means or by acid or alkaline cleaning agents at the end of the sugar campaign. These procedures are labour-intensive and give rise to corrosion.
Various agents, in particular polycarboxylic acids and polyphosphonic acids and mixtures thereof, have been proposed to prevent calcium scale when concentrating sugar streams. Examples are a mixture of polyacrylic acid and sodium hexametaphosphate U.S. Pat. No. 3,483,033), partially hydrolysed polyacrylamide (U.S. Pat. No. 4,072,607), polymaleic acid (U.S. Pat. No. 4,452,703) and nitrilotrismethylphosphonic acid (NL 89165) and a mixture of copolymers based on acrylic acid and the like (EP 11 743). In practice polyacrylates are frequently used to prevent deposition of calcium salts during the sugar evaporation process. However, polyacrylate and the other proposed scale inhibitors have the disadvantage that solutions thereof have a relatively high viscosity and consequently do not mix readily with the sugar stream, whilst dilution of the sugar stream, to improve mixing is undesirable, inter alia because of the resultant higher energy consumption. Furthermore, polyacrylate is a synthetic polymer and acrylic acid, the monomer thereof, which is toxic to some extent, is inevitably present in the solutions. In connection with this, stringent requirements are imposed on the quantity of polyacrylates that may be used for this application. According to the, FDA, the maximum concentration of polyacrylate is 3.6 ppm on unrefined juice. Furthermore, when polyacrylates and the like are used, an anti-foam is needed for the control of foam which is produced as a consequence of the presence of proteins, betaine and saponins in the sugar stream.
No serious alternatives to polyacrylates as scale inhibitors in the sugar process have been reported thus far. A glycerol-based antiscaling agent was found to be ineffective in sugar factory evaporators (Majalah Penilitian Gula 32 (1996) 28-34).
It has now been found that carboxymethylinulin (CMI) and other carboxyalkylfructans are an alternative for polyacrylates which work well in the control of the deposition of calcium salt and other scale, such as deposits of magnesium salts, silicates and organic material, during evaporation in the sugar process. The scale-inhibiting action of CMI is comparable with or even better than that of polyacrylates, whilst CMI is biodegradable and non-toxic. Moreover, CMI can be dissolved in any desired ratio in the sugar stream without dilution, and does not affect the crystallisation of sucrose. Surprisingly, it has also been found that when CMI is used as scale-inhibiting agent, less or even no anti-foam has to be added to control troublesome foam that can be produced during and after the evaporation process. A further advantage is that, in contrast to polyacrylates, the CMI that passes into the molasses following sugar separation does not interfere with the fermentation of the molasses (to produce, for example, alcohol ad citric acid) and if it then passes via the residual stream into the vinasse it in no way prohibits the use of the latter as, for example, cattle feed.